The present invention relates to a collimator constituted by a pair of gradient index rod lenses arranged to face each other, a collimator array in which such collimators are arranged side by side at intervals of a predetermined pitch, and a method of producing those collimator and collimator array. Particularly it relates to a technique for simplifying the step of optical alignment to thereby obtain products having high performance.
In optical information transmission, there is heretofore used an optical system in which: divergent beam emitted from one optical fiber is collimated by one lens to thereby generate collimated beam; and the collimated beam is propagated and then condensed by the other lens to thereby make the condensed beam enter the other optical fiber. Such an optical system is called xe2x80x9ccollimatorxe2x80x9d. Various kinds of optical elements such as a filter, an optical isolator, an optical switch, a beam modulator, and soon, can be put between the two lenses to thereby form a diversity of optical modules.
In the above optical system, convex lenses are used as the aforementioned lenses. A collimator using gradient index rod lenses are also used. Such a gradient index rod lens has characteristic that the refractive index changes gradially in a radial direction with the axial line as its center. When the lens length of the gradient index rod lens and the distance between the lens and the optical fiber are defined in accordance with the wavelength of beam, the beam incident on the lens can be collimated or condensed by the lens so that the collimated or condensed beam can be made exit out of the lens.
FIG. 5 is a perspective view showing a single-core collimator having a pair of gradient index rod lenses 1a arranged with respective one ends facing each other. Optical fibers 4 are connected to the respective other end surfaces which are opposite to the one facing end surfaces of the gradient index rod lenses 1a. The optical axes of the optical fibers 4 are made coincident with the optical axes of the gradient index rod lenses 1a respectively. The beam from one optical fiber 4 is collimated by the gradient index rod lens 1a connected to the one optical fiber 4 to thereby generate collimated beam. The collimated beam is condensed by the other gradient index rod lens 1a and led to the other optical fiber 4 connected thereto. In this manner, an optical signal is transmitted. Therefore, in a collimator using such gradient index rod lenses 1a, the optical axes of the gradient index rod lenses 1a, 1a facing each other need to be made coincident with each other and, the optical axis of each of the gradient index rod lenses 1a needs to be made coincident with the optical axis of a corresponding optical fiber 4 accurately so as to reduce coupling loss.
However, as shown in FIGS. 6(a) and 6(b), axis displacement in various directions, generally, occurs when the gradient index rod lenses 1a, 1a are arranged so as to face each other. FIG. 6(a) is a plan view of the collimator depicted in FIG. 5 from the gradient index rod lens 1a side. FIG. 6(b) is a side view of the collimator. As shown also in FIG. 5, the reference character C designates an ideal optical axis common to the gradient index rod lenses 1a and the optical fibers 4. A direction parallel to the optical axis C is defined as a Z-direction. The direction perpendicular to the horizontal direction C is defined as an X-direction. A-vertical direction perpendicular to the. X- and Z-directions is defined as a Y-direction.
As axis displacement between the lenses facing each other, displacement in the X- and Z-directions and inclination xcex8x in the X-direction may be supposed to occur as shown in FIG. 6(a) and displacement in the Y- and Z-direction and inclination xcex8y in the Y-direction may be supposed to occur as shown in FIG. 6(b). Therefore, optical alignment of four axes is required for forming a pair of lenses. In the case of a collimator array, because a lens array which has been already fixed on one side needs to be subjected to optical alignment, rotation xcex8z around the optical axis C as shown in FIG. 6(a) is further added. That is, in a collimator array, optical alignment of six axes needs to be repeated for the respective lens elements 1a, 1a. 
As described above, in the background art, complex and delicate optical alignment work is required in multiaxial directions between the gradient index rod lenses 1a, 1a. Particularly in a collimator array, the optical alignment work must be repeated by a plurality of times, so that the optical alignment work is very difficult.
The present invention is devised upon such circumstances and an object of the invention is to provide a collimator or collimator array in which the number of axes to be optically aligned between gradient index rod lenses is reduced, and a method of producing such a collimator or collimator array.
In order to achieve the above object, according to the present invention, there is provided a collimator constituted by a pair of gradient index rod lenses arranged to face each other, wherein; the pair of gradient index rod lenses are fixed respectively on a pair of substrates each having end surfaces forming planes so that an optical axis of each gradient index rod lens is parallel to one of the end surfaces of corresponding one of the substrates; and the pair of substrates are arranged to face each other while the end surfaces of the substrates are kept parallel to each other respectively and correspondingly. Further, there is provided a collimator array constituted by a pair of rod lens arrays arranged to face each other, each of the rod lens arrays including gradient index rod lenses arranged side by side at intervals of a predetermined pitch, wherein: the gradient index rod lenses in each of the rod lens arrays are fixed on corresponding one of a pair of substrates having end surfaces each forming a plane so that optical axes of the gradient index rod lenses are arranged at intervals of the predetermined pitch and parallel to one of the end surfaces of the corresponding substrate; and the pair of substrates are arranged to face each other while the end surfaces of the substrates are kept parallel to each other.
In order to achieve the above object, according to the present invention, there is provided a method of producing a collimator having a pair of gradient index rod lenses arranged to face each other, constituted by the steps of: fixing a long-size gradient index rod lens raw material on a substrate having end surfaces each forming a plane so that an optical axis of the gradient index rod lens raw material is parallel to one of the end surfaces of the substrate; cutting the substrate provided with the gradient index rod lens raw material at a predetermined position in a plane perpendicular to the optical axis of the gradient index rod lens raw material to thereby divide the substrate provided with the gradient index rod lens raw material into two lens parts; adjusting the lens parts to make each of the lens parts have a defined lens length; and arranging the two lens parts so that the cut end surfaces of the lens parts are made to face each other while optical axes of the two lenses are made coincident with each other.
Further, in the method of producing a collimator, the pair of substrates after cutting are made to face each other while end surfaces of the substrates are kept parallel to each other respectively to thereby make the optical axes of the pair of lenses coincident with each other.
In order to achieve the above object, according to the present invention, there is provided a method of producing a collimator array having a pair of rod lens arrays arranged to face each other, each of the rod lens arrays having gradient index rod lenses arranged side by side at intervals of a predetermined pitch, constituted by the steps of: fixing long-size gradient index rod lens raw materials on a substrate having end surfaces each forming a plane so that the long-size gradient index rod lens raw materials are arranged side by side at intervals of the predetermined pitch while optical axes of the gradient index rod lens raw materials are parallel to one of the end surfaces of the substrate; cutting the substrate provided with the gradient index rod lens raw materials at a predetermined position in a plane perpendicular to the optical axes of the gradient index rod lens raw materials to thereby divide the substrate provided with the gradient index rod lens raw materials into lens array parts; adjusting the divided lens array parts so that each of the gradient index rod lens raw materials in the lens array parts has a defined lens length; and arranging the lens array parts so that the cut end surfaces of the lens array parts are made to face each other while optical axes of the lens parts facing each other correspondingly are made coincident with each other.
Further, in the method of producing a collimator array, the pair of substrates after cutting are arranged to face each other while end surfaces of the substrates are kept parallel to each other respectively to thereby make optical axes of at least one pair of facing lenses coincident with each other.
The present disclosure relates to the subject matter contained in Japanese patent application No. 2000-306343 (filed on Oct. 5, 2000), which is expressly incorporated herein by reference in its entirety.